This invention relates to a lithographic printing method based on digital plate making performed on a computer-to-cylinder type printing apparatus, and more specifically, to the method of plate making and printing which is based on the use of an oil-based inkjet ink, achieving an excellent plate image quality as well as an excellent print quality. This invention also relates to a computer-to-cylinder type printing apparatus carrying out such procedures.
In the conventional lithographic printing, an ink-receptive area and an ink-repulsive area are formed on the surface of a printing plate, and printing ink is fed on the plate so as to selectively adhere to the ink-receptive area. The adhering printing ink is then transferred to paper. Usually, a hydrophilic area and oleophilic (ink-receptive) area are formed on the surface of a printing plate. Then, the hydrophilic area is wetted with fountain solution to repel printing ink.
An image formation (plate making) on a printing plate precursor (plate stock) is carried out, as the most popular method, by first outputting an original image on a silver halide photographic film with an analog or digital method, through which film a photosensitive diazo resin or photopolymer-based layer is exposed to light, and then by removing non-image areas of such a photosensitive layer with an alkaline developer.
Recently, with the advance of digital image formation technology and with the demand for making printing process efficient, a variety of proposals on systems is being made which can directly outputs images on printing plate using digital image information. Such methods are often called CTP (Computer-to-plate), or DDPP (Digital Direct Printing Plate). Image forming methods suitable for CTP include those using systems based on laser exposure in light or heat mode. Some of such systems are already in practical use.
However, such plate making methods based on laser exposure suffer from an environmental drawback caused by the use of alkaline developer needed to remove background areas of the plate material after image exposure. This drawback is common to the light and heat modes.
Still other plate making methods based on laser exposure are known, which, however, require expensive and bulky apparatus. Hence, systems based on inkjet imaging are attracting attention as inkjet recording uses an inexpensive and compact image-recording apparatus.
JP-A-64-27953 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a plate making method comprising image formation with inkjet recording using an oleophilic wax ink onto a hydrophilic plate material. In the method, the plate material is used only once, but the ink ejection is consistent and thus capable of making high quality plates stably.
Further, JP-A-11-70632 discloses a plate making method based on image formation with inkjet recording using an aqueous solution or colloidal dispersion of a water-repellent organic acid salt on a hydrophilic plate material.
In the methods cited above, the printing plate must be manually loaded on the plate cylinder of a lithographic printing apparatus, thus one requires a relatively long time for plate loading, and, in the case of multi-color printing, registration error tends to occur.
Further, inkjet imaging systems are proposed which perform plate making on the printing apparatus for higher operation efficiency.
JP-A-4-97848 discloses such an on-cylinder image-recording system in which a plate drum having a hydrophilic or an oleophilic surface is used instead of the conventional plate cylinder, and in which an oleophilic or a hydrophilic image is formed with inkjet recording. The image is then used for printing, and removed or erased after printing. However, this method is disadvantageous in that the desired removability of the image (i.e., cleanability) and press life cannot be accomplished at the same time.
The inkjet image thus formed is thermally fixed. Thermal fixing has a serious effect on the press life. The conventional fixing methods based on heat emission and radiative heating with a lamp heater or a ceramic heater needed a relatively long heating time such as, for example, 20 sec at 100xc2x0 C. in order to impart a sufficient press life.
In contrast, when a heat roller is used for the present purpose, the image is not only heated by the roller, but also pushed into the surface structure of the plate by the pressure of the roller. It has been confirmed that a sufficient press life is achieved with a heating condition of 80xc2x0 C. for 1 sec.
However, still the temperature of the heat roller must be raised to 150xc2x0 C. or more to achieve such a heating condition. Then, the thermal resistance of the roller material mainly made of rubber is not enough, and the monomer or additives of the rubbery material tend to bleed under such a high temperature. Printing plates fixed with a roller in such a condition sometimes exhibited inferior plate characteristics.
The invention has been made paying attention to the above-described objects.
An object of the present invention is to provide a computer-to-cylinder type lithographic printing method and apparatus free of development processing and suitable for digital plate making.
Another object of the present invention is to provide a lithographic printing method and apparatus which can produce a large number of prints having crisp and sharp images by in a simple manner with inexpensive equipments.
A still other object of the present invention is to provide a heat-fixing member that can achieve satisfactory press life with a short-time heating, thus increasing the fixing speed and saving the space needed for fixing step.
A further still other object of the present invention is to provide a lithographic printing method and apparatus that can achieve satisfactory press life with a short-time heating without causing any bleed, thus providing printed matters of high image quality.
Other objects and effects of the invention will become apparent from the following description.
The above-described objects of the present invention have been achieved by providing the following computer-to-cylinder type lithographic printing methods and apparatuses.
(1) A method of computer-to-cylinder type lithographic printing comprising:
loading a plate material on a plate cylinder of a printing apparatus;
forming an image, based on image data signal, directly onto the plate material by an inkjet image-recording process comprising ejecting an oil-based ink from a recording head;
heat-fixing the thus formed inkjet image to prepare a printing plate; and
performing lithographic printing with the thus prepared printing plate,
wherein said heat fixing step comprises heating with a heat roller.
(2) The method of computer-to-cylinder type lithographic printing according to item (1) above, wherein said heat-fixing step further comprises preliminary heating prior to said heating with the heat roller.
(3) The method of computer-to-cylinder type lithographic printing according to item (1) or (2) above, further comprising at least one of:
removing dust present on a surface of the plate material either or both prior to and during said inkjet image formation; and
cleaning the recording head at least after the completion of said printing plate preparation.
(4) A computer-to-cylinder type lithographic printing apparatus comprising:
an image-forming unit comprising an inkjet recording device which has a recording head and which forms an image directly onto a plate material loaded on a plate cylinder by ejecting an oil-based ink from the recording head based on image data signal;
heat-fixing unit which fixes the formed image to prepare a printing plate; and
lithographic printing unit which carries out lithographic printing with the thus prepared printing plate having the heat-fixed image,
wherein said heat fixing unit comprises a heat roller.
(5) The computer-to-cylinder type lithographic printing apparatus according to item (4) above, wherein said heat fixing unit further comprises a preliminary heating member disposed at an upstream portion of said heat roller.
(6) The computer-to-cylinder type lithographic printing apparatus according to item (4) or (5) above, further comprising a distancing/approximating member capable of distancing and approximating said heat-fixing unit with respect to the plate cylinder so that said heat-fixing unit is distant from the plate cylinder except during the fixing.
(7) The computer-to-cylinder type lithographic printing apparatus according to any one of items (4) to (6) above, wherein said image forming unit further comprises a dust removing member which removes dust present on a surface of the plate surface either or both prior to and during the image formation.
(8) The computer-to-cylinder type lithographic printing apparatus according to any one of items (4) to (7) above, wherein the plate cylinder is rotatable to carry out main scanning upon the image formation.
(9) The computer-to-cylinder type lithographic printing apparatus according to item (8) above, wherein said inkjet head comprises a single channel head or a multi channel head and movable in an axial direction of the plate cylinder to carry out sub-scanning upon the image information.
(10) The computer-to-cylinder type lithographic printing apparatus according to item (8) above, wherein said recording head comprises a full-line head having a width substantially equal to the width of said cylinder.
(11) The computer-to-cylinder type lithographic printing apparatus according to any one of items (4) to (10) above, wherein said image-forming unit further comprises a head distancing/approximating member capable of approximating said recording head to said cylinder upon the image formation onto the plate material and of distancing said recording head from the cylinder except during the image formation.
(12) The computer-to-cylinder type lithographic printing apparatus according to any one of items (4) to (11) above, wherein said image-forming unit further comprises a recording head-cleaning member which cleans said recording head at least after the completion of said printing plate preparation.
(13) The computer-to-cylinder type lithographic printing apparatus according to any one of items (4) to (12) above, wherein said lithographic printing unit comprises a paper dust removing member which removes paper dust generating upon the lithographic printing.